Actuator devices of the above mentioned general type typically include a solid state electrostrictive or piezoelectric element as an actuator, which becomes elongated in a lengthwise direction due to strain induced in the electrostrictive or piezoelectric material upon application of an electrical voltage thereto. The present specification applies to both electrostrictive and piezo-electric actuator elements, which will generally be referred to as electrically strainable solid state actuator elements, which comprise corresponding electrically strainable solid state materials.
Since the electrically induced strain in electrostrictive and piezoelectric materials is rather small in relation to the voltage that is applied to the material, the resulting strain must be amplified or multiplied to provide a sufficient stroke or displacement range to be useful as an actuator in most mechanical or physical applications. For this reason, it is known to provide a laminated stack of layers of electrically strainable solid state material to form a solid state actuator element, so that the total stroke or displacement of the stack element is a series addition of the electrically induced strains of all of the respective solid state material layers. However, even the multiplied displacement achieved by such an electrostrictive or piezoelectric stack element is inadequate for many applications.
Therefore, attempts have been made to provide a mechanical linkage or transmission mechanism, and particularly a step-up transmission mechanism, for amplifying the stroke displacement provided by the solid state actuator element. U. S. Pat. No. 4,937,489 (Hattori et al.) discloses an electrostrictive actuator including at least one electrostrictive element and a transmission mechanism coupled thereto for amplifying the initial displacement of the electrostrictive element. The transmission mechanism consists of a solid and relatively massive metal plate having a groove or cut-out for receiving the actuator element therein, and a plurality of slits so as to form a substantially rectangular frame made of rigid frame members connected to each other by integral, flexible hinge joints formed by thinner slitted or notched areas of the metal plate material. The result is a linkage frame effectively made up of six rigid bodies and six hinges. An elongation of the actuator element causes a deformation of this linkage frame such that a small change in length of the electrostrictive member is kinematically converted into a multiply amplified output stroke provided by one of the linkage bodies of the actuator device.
In view of the rather high tension loads effective on the elastic hinge joints, these hinge joints must have a rather large tensile strength and stiffness, and therefore must have a correspondingly large cross-sectional area. This simultaneously causes the disadvantage that the overall bending stiffness of the bending hinge, and also the outer fiber strain resulting in the material of the hinge due to the bending movement, increase sharply with increasing cross-sectional area of the hinge joint. As a result, the hinge joints, which are appropriately dimensioned for the prevailing loads, cause an elastic return force and bending resistance that acts against the electrically induced length variation of the actuator element and therefore noticeably reduces the effective stroke of the actuator device.